A significant class of optical devices are commonly called “planar light-wave circuits” or “planar light-wave chips” or just PLCs. PLCs comprise technologies wherein optical components and networks are disposed monolithically within a stack or stacks of optical thin films supported by a common mechanical substrate such as a semiconductor or glass wafer. PLCs are typically designed to provide specific transport or routing functions for use within fiber-optic communications networks. These networks are distributed over a multitude of geographically dispersed terminations and commonly include transport between terminations via single-mode optical fiber. For a device in such a network to provide transparent management of the optical signals it must maintain the single-mode nature of the optical signal. As such, the PLCs are commonly, though not strictly, based on configurations of single-mode waveguides. Since optical signals do not require return paths, these waveguide configurations do not typically conform to the classic definition of “circuits”, but due to their physical and functional resemblance to electronic circuits, the waveguide systems are also often referred to as circuits.
The standard family of materials for PLCs, widely demonstrated to have superior loss characteristics, is based on silicon dioxide, commonly called silica. The silica stack includes layers that may be pure silica as well as layers that may be doped with other elements such as Boron, Phosphorous, Germanium, or other elements or materials. The doping permits control of index-of-refraction and other necessary physical properties of the layers. Silica, including doped silica, as well as a few less commonly used oxides of other elements, are commonly also referred to collectively as “oxides.” Furthermore, although technically the term “glass” refers to a state of matter that can be achieved by a broad spectrum of materials, it is common for “glass” to be taken to mean a clear, non crystalline material, typically SiO2 based. It is therefore also common to hear of oxide waveguides being referred to as “glass” waveguides. Subsequently, the moniker “silica” is used to refer to those silicon oxide materials suitable for making waveguides or other integrated photonic devices. It is important to note that in the context of this invention, other waveguide materials, such as lithium niobate, spin-on glasses, silicon, siliconoxynitride, silicon oxycarbide, polymers or other materials described in U.S. Pat. No. 6,614,977 (the entire content of which is hereby incorporated herein by reference), are also appropriate
In a typical example of a PLC, a waveguide formed of a core material lies between a top cladding layer and a bottom cladding layer. In some instances, a top cladding may not be used. Waveguides are often formed by at least partially removing (typically with an etching process) core material beyond the transverse limits of the channel waveguide and replacing it with at least one layer of side cladding material that has an index of refraction that is lower than that of the core material. The side cladding material is usually the same material as the top cladding material. In this example, each layer is doped in a manner such that the waveguide has a higher index of refraction than either the top cladding or bottom cladding. When layers of silica glass are used for the waveguide, the layers are typically situated on a silicon wafer. As a second example, waveguides can comprise three or more layers of InGaAsP. In this example, adjacent layers have compositions with different percentages of the constituent elements In, P, Ga, and As. As a third example, one or more of the optical layers of the waveguide may comprise an optically transparent polymer. Another example of a waveguide comprises a layer with a graded index such that the region of highest index of refraction is bounded by regions of lower indices of refraction. A doped-silica waveguide is usually preferred because it has a number of attractive properties including low cost, low loss, low birefringence, stability, and compatibility for coupling to fiber.
Many integrated optical devices require the creation of physical structures that are highly symmetric. A key example is a planar waveguide coupler consisting of two optical waveguides coupled to each other across a gap. In many cases, the achievement of highest performance in such a coupler requires the two waveguides to be identical to each other. A conventional approach to achieving this goal is to define the optical waveguides using a photolithography and etching process with a pattern consisting of two waveguides of identical cross section separated by a gap. In this scheme, the fabrication of identical guides relies upon very high fidelity in the lithography and etching processes to reproduce the identical mask patterns into the optical waveguides. This strategy will allow integrated optical couplers to be fabricated with a certain level of performance that may be adequate for some types of devices. However, the ultimate performance may be limited due to asymmetries induced by the fabrication process.